The so-called key insertion technique, by which a part of one television picture is inserted into another picture to produce a special effect, is frequently used in television broadcasting. One example of such a technique is chroma-key insertion wherein a part of a first picture is designated by a chroma-key signal produced from a second picture and the designated part is inserted into the second picture. However, since a chroma-key signal undergoes a change in position and dimension with the movement of a television camera, employed for the pickup of the second picture, the shooting angle of the camera must be modified accordingly. This involves serious difficulties and to eliminate the difficulties a method has been proposed in which the picture to be inserted is compressed in accordance with the chroma-key signal. (See for example Japanese Patent Publication No. 53-9896). With this technique, the position and dimensions of the picture are determined by comparing the chroma-key frame with a standard television frame.
Referring to FIGS. 1A to 1E, it is assumed that the picture of FIG. 1B is compressed into size of a chroma-key frame shown in FIG. 1C to produce a picture (FIG. 1D) and that the picture of FIG. 1D is inserted into the picture of FIG. 1A to produce the picture of FIG. 1E. In this example, the chroma-key signal representing the chroma-key frame of FIG. 1C serves as the standard signal to designate into what position and to what demensions the picture of FIG. 1B should be compressed. Generally, the chroma-key signal of FIG. 1C is produced by mixing in appropriate proportions of the blue component as a main constituent, out of the various chromatic components (the red, green and blue components) constituting the picture of FIG. 1A, with the two other components. It is therefore customary, when a chroma-key signal is to be produced, to compose the picture of FIG. 1A by leaving blue that part of the picture which is cross-hatched.
Since the aspect ratio (the ratio of the width to the height of a frame) of a television picture is generally prescribed to be 4:3, that of the chroma-key frame of FIG. 1C is naturally set at 4:3. However, when the television camera which picks up picture A, shoots in an oblique direction with respect to the chroma-key frame of FIG. 1C, the aspect ratio of the chroma-key frame will change. This is especially true when the camera is laterally moved and the result is that the chroma-key frame of FIG. 1C, in the picture, may greatly deviate from the standard aspect ratio as shown in FIGS. 2A and 2B. This results in a disadvantage in that the video signal, compressed and inserted into the chroma-key frame, does not have an aspect ratio of 4.3, resulting in an awkward appearing picture. The positional relationship between the chroma-key frame, and the picture to be compressed and inserted, should desirably be such that the picture to be inserted is equally compressed for both height and width. The picture should be without distortion in aspect ratio, as illustrated in FIGS. 2C and 2D and appear as if it were in fact pasted on the chroma-key-frame.
It is therefore an object of the present invention to provide a chroma-key tracking system which, even when the chroma-key frame deviates from the standard aspect ratio, will compress a picture to be inserted into the chroma-key frame with the prescribed aspect ratio.